Employee retirement benefits - Components of Net Periodic Benefit Cost (Detail) - CAD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Pension benefits:
Service cost	$ 46	$ 41
Interest cost	91	93
Expected return on plan assets	(113)	(93)
Amortization of prior service cost	7	4
Amortization of actuarial loss (gain)	12	11
Net benefit cost	43	56
Other postretirement benefits:
Service cost	4	3
Interest cost	6	7
Amortization of actuarial loss (gain)	(2)	(2)
Net benefit cost	$ 8	$ 8